Many operations carried out in refineries and petrochemical plants involve feeding a hydrocarbon/hydrogen stream to a reactor, withdrawing a reactor effluent stream of different hydrocarbon/hydrogen composition, separating the effluent into liquid and vapor portions, and recirculating part of the vapor stream to the reactor, so as to reuse unreacted hydrogen.
Reactions carried out under such a scheme can be divided generally into hydrogen-consuming reactions and hydrogen-producing reactions. The hydrogen-consuming reactions include hydrotreating, hydrocracking and various hydrogenation operations. The hydrogen-producing reactions include reforming and various dehydrogenation operations. Of these, the principal hydrogen producer for the refinery is reforming, and an important aspect of reformer operation is to generate as much hydrogen as possible, consistent with other requirements, of a quality suitable for use in the hydrogen-consuming units, particularly hydrocrackers and hydrotreaters.
The reformer is the unit that provides the octane level needed for the gasoline product of the refinery. The reformer feedstock is straight run naphtha or a naphtha cut from other refinery operations, such as coker naphtha or FCC (fluid catalytic cracking) naphtha. Although the bulk of components in straight run naphtha are paraffins, also present may be naphthenes, aromatics and olefins; non-virgin naphtha stocks tend to be higher in aromatics and olefins. Reforming involves the upgrading of these components by various reactions. The process is generally carried out in three reaction zones, in each of which specific reactions are favored. For example, the first zone may perform, among other reactions, dehydrogenation of methylcyclohexane to toluene (RON 120), the second zone may perform dehydrocyclization of iso-heptane to toluene, and the third zone may perform isomerization of normal to iso-heptane (RON 90), as well as cracking of n-heptane to pentane (RON 90). Although the process is an overall producer of hydrogen, hydrogen is recycled back to the feed to maintain the hydrogen-to-hydrocarbon ratio in the reactors within a range to favor the desired reactions and to prolong the catalyst life. Typically hydrogen:hydrocarbon molar ratios up to about 10:1 are used.
In an ideal situation, no cracking that results in light hydrocarbons would take place, and the only by-product of the reactions would be hydrogen. In practice, this cannot be achieved. Conversion of iso-heptane to toluene and of methylcyclohexane to toluene are both hydrogen-producing reactions, but the cracking reactions result in formation of light hydrocarbon fragments, such as methane, ethane, propane and butanes. These light hydrocarbons contaminate the hydrogen product and may result in over-production of fuel gas. Under some reforming conditions, even more undesirable side reactions can give rise to formation of polycyclic aromatic compounds, some of which can be carried into the recycle gas. The net result is that, in an average refinery, 20,000 bbl of straight run naphtha feedstock may be converted into about 15,000 bbl of reformate and 5,000 bbl of gas, which includes substantial amounts of C.sub.1 -C.sub.5 hydrocarbons.
The gaseous effluent from the reactor series is cooled and separated into liquid and vapor phases. The phase separation into liquid and vapor portions is often carried out in one or more steps by simply changing the pressure and/or temperature of the effluent. Therefore, in addition to hydrogen, the overhead vapor from the phase separation usually contains light hydrocarbons, particularly methane and ethane. In a closed recycle loop, these components build up, change the reactor equilibrium conditions and can lead to reduced product yield. This build-up of undesirable contaminants is usually controlled by purging a part of the vapor stream from the loop. Such a purge operation is unselective however, and, since the purge stream may contain as much as 80 vol % or more hydrogen, multiple volumes of hydrogen can be lost from the loop for every volume of contaminant that is purged.
Since the reformer is a net hydrogen producer, the overhead vapor is typically split into at least two portions, one for recycle in the reactor loop, the other that forms a purge from the loop and that is frequently submitted to additional separation and treatment. This creates a net hydrogen stream of a relatively high hydrogen concentration, such as above 80% or 90%, for use elsewhere in the plant, and a light hydrocarbon stream to be sent for light ends recovery or to the fuel header.
Hydrogen recovery techniques that have been deployed in refineries include, besides simple phase separation of fluids, pressure swing adsorption (PSA) and membrane separation. U.S. Pat. No. 4,362,613, to Monsanto, describes a process for treating the vapor phase from a high-pressure separator in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the hydrocracker reactor. U.S. Pat. No. 4,367,135, also to Monsanto, describes a process in which effluent from a low-pressure separator is treated to recover hydrogen using the same type of hydrogen-selective membrane. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane again permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L'Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from cracking effluent, the hydrogen separation being accomplished by a hydrogen-selective membrane. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and light hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal. U.S. Pat. No. 5,332,492, to UOP, concerns treatment of effluent gases from catalytic reformers by cooling to between -9.degree. C. and -26.degree. C. followed by PSA.
The use of certain polymeric membranes to treat off-gas streams in refineries is also described in the following papers: "Prism.TM. Separators Optimize Hydrocracker Hydrogen", by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; and "Optimizing Hydrocracker Hydrogen" by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. The use of membranes in refinery separations is also mentioned in "Hydrogen Technologies to Meet Refiners' Future Needs", by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons. Papers that specifically concern treatment of reformer off-gases are "Hydrogen Purification with Cellulose Acetate Membranes", by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; and "Plant Uses Membrane Separation", by H. Yamashiro et al., in Hydrocarbon Processing, February 1985. In these papers, a system and process using membranes to treat the overhead gas stream from the absorber/recontactor section of the plant are described. All of these papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons.
A chapter in "Polymeric Gas Separation Membranes", D. R. Paul et al. (Eds.) entitled "Commercial and Practical Aspects of Gas Separation Membranes", by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. Nos. 4,654,063 and 4,836,833, to Air Products, and U.S. Pat. No. 4,892,564, to Cooley.
U.S. Pat. No. 5,332,424, to AirProducts, describes fractionation of a gas stream containing light hydrocarbons and hydrogen using an "adsorbent membrane". The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,435,836; 5,447,559 and 5,507,856, which all relate to purification of streams from steam reformers. U.S. Pat. No. 5,634,354, to Air Products, discloses removal of hydrogen from hydrogen/olefin streams. In this case, the membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen.
U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.